I successfully transferred my 20w16a world into 20w17a, and it shortly began to rain. I am using the Tunneler's Dream superflat preset and at about y=200 (unsure if y value affects glitch) I could see and hear the rain as I would normally above ground, despite being under many layers of stone. The rain particles only appeared in lines parallel to the x-axis spaced every 4 blocks along the z-axis.

Additionally, it seems that whenever I use /weather clear, the rain restarts soon after. This might be a feature, but I find it annoying nonetheless.